BIND 10 #1809: support NSEC for NXDOMAIN in in-memory

[BIND 10 Development]

#1809: support NSEC for NXDOMAIN in in-memory
-------------------------------------+-------------------------------------
            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  New
           Component:  data source   |  Tasks
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:  in-
         Total Hours:  0             |  memory NSEC
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
-------------------------------------+-------------------------------------
 We'll update this case of InMemoryZoneFinder::find().

 {{{#!c++
                 // fall through
             case DomainTree::NOTFOUND:
                 LOG_DEBUG(logger, DBG_TRACE_DATA, DATASRC_MEM_NOT_FOUND).
                     arg(name);
                 return (createFindResult(NXDOMAIN, ConstRBNodeRRsetPtr(),
                                          false));
 }}}

 It uses getClosestNSEC() to get the NSEC for the previous name of the
 non-existent name and includes it in the answer.  Note that it
 shouldn't do this extra work unless DNSSEC is required and the zone is
 known to be NSEC-signed (because the cost is not zero).

 It dpends on #1802, #1803, #1804, dnd #1805.

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Ticket URL: <http://bind10.isc.org/ticket/1809>
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BIND 10 Development